The U.S. Chamber of Commerce is poised to unveil an onslaught of general-election advertising this week, launching TV spots in nearly a dozen of the most competitive House districts in the 2014 midterm elections.

The heavily funded, Republican-friendly group also has reserved airtime in the Kentucky and Colorado Senate races. The Colorado ads are expected to feature Florida Sen. Marco Rubio, whose PAC has endorsed Republican candidate Cory Gardner.

The Chamber declined to share the cost of the TV campaign but called it a multimillion dollar effort, and it appears to represent the powerful business lobby’s most intensive effort so far to leave a mark on the congressional landscape.

Officials with the Chamber said the campaign will include 11 districts from California to Massachusetts. The races feature vulnerable Democratic incumbents, Republican lawmakers targeted in the general election and one GOP congressman, Idaho’s Mike Simpson, who is facing a primary challenge from the right. During primary season, the Chamber has regularly favored the GOP’s establishment wing over activist, tea party candidates.

Chamber spokeswoman Blair Holmes said the new campaign “reinforces the Chamber’s commitment to maintaining the pro-growth majority in the House, which is our top political goal this cycle.”

All of the ads are positive, Holmes said, “and feature candidates who believe in economic growth and the free enterprise system, who know how to govern and who can win in November.”

The Chamber’s fresh ads are to begin Wednesday. In addition to Simpson in Idaho, the commercials are intended to bolster Republican incumbents David Valadao of California, Mike Coffman of Colorado, Andy Barr of Kentucky, Dan Benishek of Michigan, Joe Heck of Nevada and Chris Gibson of New York, all of whom face credible Democratic challengers on the November ballot.

The group also plans to air commercials in support of four Republican candidates running against Democratic members of Congress. The GOP challengers include Stewart Mills of Minnesota, who is challenging Democratic Rep. Rick Nolan; former Rep. Bob Dold of Illinois, who is seeking to take back his old seat from Democratic Rep. Brad Schneider; and Richard Tisei of Massachusetts, who is challenging Democratic Rep. John Tierney.

California congressional candidate Doug Ose, a former congressman battling a conservative primary opponent for the right to take on Democratic Congressman Ami Bera, also will get a boost from the national Chamber.

“As a small business leader, Doug Ose created jobs in Northern California. As our congressman, Doug Ose fought for families and jobs,” says the ad set to air in California’s 7th District. “We need a leader in Washington who gets things done. We need Doug Ose: a local leader with a proven record.”

The Chamber’s expanding political map also appears to mark a new phase in the organization’s 2014 activity, aimed more at shoring up general-election favorites than litigating the GOP’s internal fights.

The national group already has played a significant role in multiple GOP primaries, backing Simpson in Idaho and Mississippi Sen. Thad Cochran in incumbent-versus-challenger nomination fights, and boosting favored candidates for Senate seats in Georgia and North Carolina.

The Chamber-endorsed candidate for Senate in North Carolina, House Speaker Thom Tillis, has led the polls ahead of Tuesday’s primary there. Should he capture the GOP nomination without a runoff, that would represent an early primary-season win for the Chamber — and provide a helpful backdrop for general-election ads starting Wednesday.